I'm a Windows guy, so take this with a grain of salt. But its possible that the version of Mac OS doesn't recognize NTFS or FAT32 storage. So you may need to re-format the USB to EX4 so that your Mac can read it correctly. This would need to be done on a Linux computer, because Windows doesn't read/write to EX4, and Mac doesn't read/write to NTFS. But Linux does both so that you can take the data off before you format it, and put it back on after.